Conventional operational amplifiers use both a PMOS transistor pair and a NMOS transistor pair to alternate functioning so as to ensure that the operational amplifier can be operating when the input signal ranges from ground to the positive power supply (vdd). However, there are cases when both the PMOS transistor pair and the NMOS transistor are functioning, or neither the PMOS transistor pair nor the NMOS transistor pair is functioning, which means a changing transconductance of the operational amplifier and resulting in a distortion of output signal. Therefore, it is desirable to design an operational amplifier with substantially constant transconductance.